Image Resolution

I can’t count the number of times someone has said to me, “I found this photo on the Web and want to use it in my document, but I can’t get it to print clearly. How do I fix it?”

I hate that question, because no matter how much I want to help, I always have to give the same answer: “You can’t.”

Why not?

Computer screens are low resolution devices. Printers are high resolution devices. If you take a file designed for a low resolution device and send it to a high resolution device, the result is going to be blurry or grainy.

To understand why, you have to understand image resolution.

Digital photos are made up of colored dots, called pixels. Image resolution is simply the number of dots/pixels per horizontal inch of photo. Web images always have a resolution of 72 dpi (dots per inch), also written as 72 ppi (pixels per inch).

On a computer screen, 72 dpi is sufficient to make a photo appear crisp and clear. But when you print the same photo, suddenly you can distinguish individual pixels. That’s because although printers use colored dots as well, their dots are MUCH smaller than pixels. To print a photo with the same clarity as it appears on the Web, you would need to print it at approximately ¼ the size of the original. Otherwise, the printer has to stretch out each dot, resulting in a blurry photo.

Admittedly, this is a gross oversimplification of what’s actually happening, but I hope it’s good enough to convey the most important point: If you want an on-screen photo and a printed photo to have the same level of clarity at the same size, the printed version has to have a LOT more pixels.

How many more pixels?

As I mentioned above, Web images always have a resolution of 72 dpi. Printed images typically need a resolution of 200 to 300 dpi. That means that a print image needs to have pixel dimensions approximately 3 to 4 times larger than a web image.

72 dpi x 3 = 216 dpi
72 dpi x 4 = 288 dpi

Pixels and Print Size

To know if your image is large enough to print well, you need to look at the pixel dimensions.

Say you want to print an image that’s 1000 pixels wide. How large can you print it without losing quality?

If you’re going to professionally print the image, you need a dpi of around 300. To calculate the maximum print size, divide the pixel width (1000) by the desired dpi (300).

1000 / 300 = 3.33

The maximum print size of the image is 3.33 inches wide.

If you’re printing the image on a desktop printer, 200 dpi is usually enough.

1000 / 200 = 5

So you could print the image at 5 inches wide.

But can’t you just sharpen the image in Photoshop instead?

Don’t I wish! People often assume there’s some magical tool in Photoshop that can convert a blurry, low-quality image into a crisp, clear one. But I assure you, there is no such tool. Yes, Photoshop has many tools for improving the quality of images, and a skilled photo editor can sometimes trick your eye into thinking a photo is much higher quality than it really is. But reducing an image to web size deletes a huge amount of detail, and it’s just not possible to restore that detail once it has been lost.

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